Dimethyl-2,6-naphthalenedicarboxylate, or "NDC," is representative of a monomer that can be used to prepare a variety of polyester materials. For example, NDC can be condensed with ethylene glycol to form poly(ethylene-2,6-naphthalate), or "PEN," a high performance polyester material.
Fibers and films made from PEN have considerably improved strength and superior thermal properties relative to films and fibers made from poly(ethyleneterephthalate). PEN therefore is an exceptional material for preparing commercial articles such as thin films used for the manufacture of magnetic recording tape and electronic components. Additionally, because of PEN's superior resistance to the diffusion of gases such as carbon dioxide, oxygen and water vapor, films made from PEN particularly are useful for manufacturing articles such as "hot fill" food containers. PEN also is useful for preparing high strength fibers which can be used to manufacture items such as tire cord.
Processes for manufacturing NDC and other aromatic esters from aromatic acids are well known. For example, U.S. Pat. Nos. 5,254,719, 5,262,560 and 5,350,874 describe processes for manufacturing NDC from 2,6-naphthalenedicarboxylic acid or "NDA" by reacting NDA with methanol. These and other processes for manufacturing NDC and other aromatic esters typically involve one or more ester crystallization or recrystallization steps, as well as a distillation step where the aromatic ester is distilled, typically using a fractional distillation column, to prepare high purity esters suitable for preparing PEN and other polyesters.
Aromatic acids useful for preparing aromatic esters can be prepared in a number of ways. For example, NDA advantageously is obtained by oxidizing a suitable naphthalenic feedstock such as 2,6-dimethylnaphthalene. Such oxidation reactions typically are conducted in a liquid phase mixture using one or more heavy metal catalysts to catalyze the oxidation of the naphthalenic feedstock to NDA. One preferred method uses a mixture of cobalt and manganese catalyst metals in a liquid phase oxidation of 2,6-dimethylnaphthalene. This method uses a low molecular weight acid such as acetic acid as the reaction solvent and air as the source of oxygen for oxidizing the methyl groups on 2,6-dimethyinaphthalene to the carboxylic acid groups of NDA. One such process is discussed in detail in U.S. Pat. No. 5,183,933 to Harper et al., the disclosure of which is hereby incorporated by reference.
We have discovered that when NDA is prepared by such an oxidation process, the catalyst metals such as cobalt and manganese can cause severe fouling of the process equipment used to manufacture NDC from NDA. Process equipment exhibiting such fouling includes heat exchangers used to increase the temperature of a mixture of NDA and methanol for a subsequent esterification reaction, heat exchangers used to increase the temperature of a mixture of NDC, NDA, and methanol for a subsequent recrystallization, filter cloths on equipment used to recover NDC particles from methanol, heat exchangers used to heat and evaporate filtration mother liquor to recover solvent, the internal portions of the reactor and associated piping used in the esterification process, and the internals of NDC distillation columns.
Equipment such as that noted above may become encrusted or fouled with solid deposits which reduce the efficiency of the equipment. If operations continue under conditions that permit fouling, the equipment can become completely plugged or otherwise inoperative. Fouled equipment can substantially reduce plant production rates. Furthermore, when fouling becomes severe, production of NDC must be stopped to clean out the fouled equipment. Thus, manufacturers of aromatic materials such as NDC require a method to reduce or eliminate fouling of process equipment. Our invention provides such a method.